Protein Misfolding and Aggregation
Proteins can fold into complex and close-packed structures. Folding is not only crucial for biological activity but failure of proteins to fold properly or remain folded can give rise to disease (reviewed in 48). Misfolding can in some cases cause protein aggregation which can further give rise to discrete deposits extracellularly (e.g., plaques) or intracellularly (e.g., inclusions in the cytosol or nucleus).
Neurodegenerative diseases such as Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease (HD), amyotrophic lateral sclerosis (ALS) and prion diseases are characterized by neural deposits of misfolded aggregated protein (reviewed in 49).
Neurodegenerative diseases, such as Alzheimer's disease (AD), Huntington's disease, amyotrophic lateral sclerosis (ALS) and Parkinson's disease/Lewy body dementia (PD, LBD) also pose major challenges to our aging population and health care system.
Sporadic AD, ALS, and PD/LBD are all associated with neural accumulation of pathological multimers of misfolded polypeptides (these could potentially be fibrils, protofilaments, and amorphous aggregates), including the amyloid-beta (Abeta) fragment of the amyloid precursor protein (APP) in AD; superoxide dismutase-1 (SOD1) in ALS, AD, and PD, and alpha-synuclein in PD and LBD. Additionally familial amyloidotic polyneuropathy (FAP) results from the aggregation of transthyretin to form amyloid deposits. As with prion diseases, mutations in genes encoding these polypeptides are associated with autosomal dominant familial forms of AD, ALS, and PD.
ALS and Protein Misfolding
Amyotrophic lateral sclerosis (ALS) is a fatal neuromuscular disease which afflicts about 30,000 patients in North America, with 5,000 new cases per year. In ALS, also known as “Lou Gehrig's disease,” muscles of the limbs, speech and swallowing, and respiration weaken and atrophy, due to degeneration of motor nerve cells that supply them from the spinal cord and brain. Half of affected patients are dead within 3 years, with survival over 5 years being less than 20%.
ALS belongs to a family of fatal neurodegenerative disorders, which includes prion illnesses, Alzheimer's and Parkinson's diseases, and in which aggregated misfolded proteins are thought to cause progressive killing of brain cells. About 20% of familial (inherited) ALS is associated with mutations in the gene encoding superoxide dismutase 1 (SOD1), an intracellular free radical defense enzyme (see 73 and Table 1 for listing of known mutations). Intracellular deposits of aggregated misfolded SOD1 have been observed in familial ALS, and also in the more common non-familial (sporadic) ALS, suggesting that SOD1 aggregation may underlie all ALS.
Experiments performed in cell culture and mice transgenic for human mutant SOD1 have established that extracellular misfolded SOD1 is highly toxic for motor neurons (1), in part by activation of killing pathways by local immune cells (microglia). Recently, it has also become clear that misfolded SOD1 is exported from the cell by both secretory and constitutive mechanisms (1, 2).
Aggregation of SOD1 may progress through a protein-based template-directed misfolding mechanism (3) similar to that proposed for the prion diseases (4). Thus, misfolded SOD1 in the extracellular space is not only directly toxic for motor neurons, but may also participate in the cell-to-cell propagation of disease throughout the nervous system by a prion-like templated misfolding process.
TABLE 1Detected Mutations in SOD1 in FALS.AminoAcidMutation4A −> S (in FALS).4A −> T (in FALS).4A −> V (in FALS).6C −> F (in FALS).7V −> E (in FALS).8L −> Q (in FALS).8L −> V (in FALS).12G −> R (in FALS).14V −> G (in FALS).14V −> M (in FALS).16G −> S (in ALS).21E −> G (in FALS).21E −> K (in FALS).37G −> R (in FALS.38L −> R (in FALS).38L −> V (in FALS).41G −> D (in FALS).41G −> S (in FALS).43H −> R (in FALS).45F −> C (in FALS).46H −> R (in FALS).48H −> Q (in FALS).49E −> K (in FALS).65N −> S (in FALS).67L −> R (in FALS).72G −> S (in FALS).76D −> Y (in FALS).80H −> A (in ALS).84L −> F (in FALS).84L −> V (in FALS).85G −> R (in FALS).86N −> S (in FALS).89A −> V (in FALS).90D −> A (in FALS).90D −> V (in FALS).93G −> A (in FALS).93G −> C (in FALS).93G −> D (in FALS).93G −> R (in FALS).93G −> V (in FALS).100E −> G (in FALS).100E −> K (in FALS).101D −> G (in FALS).101D −> N (in FALS).104I −> F (in FALS).105S −> L (in FALS).106L −> V (in FALS).108G −> V (in FALS).112I −> M (in FALS).112I −> T (in FALS).113I −> T (in FALS).114G −> A (in FALS).115R −> G (in FALS).118V −> VFLQ (in FALS).124D −> V (in FALS).125D −> H (in FALS).126L −> S (in FALS).133Missing (in ALS).134S −> N (in FALS).139N −> K (in FALS).144L −> F (in FALS).144L −> S (in FALS).145A −> T (in FALS).146C −> R (in FALS).148V −> G (in FALS).148V −> I (in FALS).149I −> T (in FALS).151I −> T (in FALS).Alzheimer's Disease
AD is a common dementing (disordered memory and cognition) neurodegenerative disease associated with brain accumulation of extracellular plaques composed predominantly of the Abeta (1-40), Abeta (1-42) and Abeta (1-43) peptides, all of which are proteolytic products of APP (reviewed in 50) In addition, neurofibrillary tangles, composed principally of abnormally phosphorylated tau protein (a neuronal microtubule-associated protein), accumulate intracellularly in dying neurons (reviewed in 49). Familial forms of AD can be caused by mutations in the APP gene, or in the presenilin 1 or 2 genes (reviewed in 51), the protein products of which are implicated in the processing of APP to Abeta. Apolipoprotein E allelic variants also influence the age at onset of both sporadic and familial forms of AD (reviewed in 52). Abeta, tau and phosphorylated tau has been detected in the blood and CSF of AD patients and in normal controls (53-55). Immunization of Alzheimer's disease patients with Abeta has shown some promising preliminary treatment results, although limited by autoimmune meningoencephalitis in humans (56-58)
Parkinson's Disease
PD is a neurodegenerative movement disorder, second only to AD in prevalence (˜350 per 100,000 population; reviewed in 59). It is clinically characterized by rigidity, slowness of movement, and tremor. Most cases of Parkinson's disease are sporadic, but both sporadic and familial forms of the disease are characterized by intracellular Lewy bodies in dying neurons of the substantia nigra, a population of midbrain neurons (60,000) that are selectively decimated in PD. Lewy bodies are predominantly composed of alpha-synuclein (60). Mutations in, and duplication of, the gene encoding alpha-synuclein have been found in patients with familial Parkinson's disease (reviewed in 61). Another gene associated with autosomal recessive PD is parkin, which is involved in alpha-synuclein degradation (61). Diffuse cortical Lewy bodies composed of alpha-synuclein are observed in Lewy body disease (LBD), a dementing syndrome associated with parkinsonian tone changes, hallucinations, and rapid symptom fluctuation (62). LBD may be the second most common form of neurodegenerative dementia after AD, accounting for 20 to 30 percent of cases among persons over the age of 60 years. Similar to the vaccine approach to Alzheimer's disease (58-60) promising results in a mouse model of Parkinson's/Lewy body disease have been obtained by immunization with alpha synuclein (63). Other dementing syndromes include fronto-termporal dementias, Pick's disease, and corticobasal dementia, and others known to neurological medicine.
SOD1 has been Detected in AD and PD Protein Aggregates
Oxidative stress has been implicated in several neurodegenerative diseases, including ALS, PD and AD. Reactive oxygen and nitrogen species (ROS and RNS respectively) generated in these environments may participate in cell injury including the abnormal oxidation of proteins or lipids. Other pathological hallmarks of such disease include cytoskeletal debris accumulations and selective neuronal death, frequently attributed to oxidative stress and the accumulated insoluble protein (74-81). Several enzymes including SOD1 have antioxidant roles. Alterations in the activity of such enzymes may contribute to a neurodegenerative disease state.
Recently, Choi et al. (64) reported that SOD1 is a major target of oxidative damage in AD and PD brains. They noted that the total level of SOD1 is increased in both AD and PD and that SOD1 forms proteinaceous aggregates that are associated with amyloid senile plaques and neurofibrillary tangles in AD brains. Choi et al. (64) have suggested that AD, PD and ALS may share a common pathogenic mechanism. It has also recently been shown that SOD1 is secreted into the extracellular space, in a form which is toxic to neurons, but more accessible by extracellular therapeutic agents (1).
The implication that extracellular misfolded SOD1 plays a role in ALS pathogenesis provides an opportunity for the antibody treatment of neurodegenerative diseases, as this compartment is accessible to antibody neutralization. In normal humans, IgGs can cross the blood brain barrier to levels between 1/100 and 1/1000 that of circulating concentrations; and transudation of immunoglobulins is often increased in diseases affecting the blood brain barrier. However, treatment of human patients with antibodies or vaccines targeted to accessible extracellular epitopes on ubiquitous proteins may lead to deleterious autoimmune effects such as those seen with Abeta in Alzheimer disease. Thus, there remains a need in the art for compositions and methods for diagnosis and treatment of misfolded SOD1-related diseases, such as ALS, AD and PD.